The conventional process of papermaking involves formation of a web of fibers on a papermaking machine using a moving porous formations support wherein water is drained from a dilute slurry of fibers deposited on the support with further water removal from the web in a press roll section and final removal of water in a dryer section of the machine. In a typical papermaking process, the fibrous web from the press roll section contains about 32 to 45 wt. % solids. The solids may include wood pulp and/or synthetic fibers along with various additives such as sizing agents, binders, fillers, pigments and the like. The wet web is then passed through a series of internally heated rolls or steam-filled cylinders in the dryer section where the web is dried to about 94% to about 99% solids content by weight. The number of drying cylinders in the dryer section is determined by the amount of water to be evaporated based on a typical evaporation rate of about 3 to about 5 pounds per hour per square foot of total dryer surface.
In the dryer section of the paper machine, water is removed from the web mainly by evaporation. Typically, the wet web is alternately contacted on its opposite sides in serpentine fashion with a series of heated co-rotating cylinders to heat the web to a temperature sufficient to evaporate water from the web to the desired dryness.
Once dried, the paper or paperboard is often further treated to improve various properties such as smoothness, gloss, wet strength and folding endurance. This further treatment may include adjusting the moisture content of the dried web, densification on high pressure rolls, calendering and/or heat-treating the product.
Various problems have persisted in the drying and calendering of paper webs on large, high-capacity paper machines. For example, drying and calendering of the products remains a high energy, capital intensive operation. Hence, the industry is challenged to develop newer and more energy efficient drying and calendering techniques. Such techniques include high-intensity drying methods where high temperatures and mechanical pressure is applied to the web during drying. Examples of currently used high-intensity drying techniques include press drying, impulse drying, and thermal/vacuum drying. However, the use of high temperature dryers and/or impulse dryers has led to additional problems such as delamination of multiply board products.
Furthermore, in the presently used drying and calendering processes, the paper may shrink in width by as much as 5 to 6% which can lead to a significant reduction in the overall production rate, and adversely affect product quality.
Accordingly, even with modern, state-of-the-art drying and calendering techniques, there remains a need to further improve the drying and calendering of paper and paperboard products to reduce energy costs and limit paper shrinkage without adversely affecting the physical properties of the finished product.
Uneven drying and streaking are other problems which have persisted in production of paper and paperboard webs. The weight and moisture irregularity of the fiber web before drying and calendering, irregularities in the heat transfer from the cylinders, edge effects and variations in the ventilation of the papermaking machine all tend to cause nonuniform drying in the cross-machine direction of the product. Such nonuniformity of drying can lead to further adverse effects on paper quality and increased waste.
U.S. Pat. No. 4,378,639 to Walker and U.S. Pat. No. 4,474,643 to Lindblad propose a solution to the problem of uneven drying across the width of the web by periodically spraying water on the web in selected areas across width of the web where low moisture or dry streaks have been detected. Because the water sprays are intermittent and used only when necessary to prevent streaks, such techniques do not effectively increase the drying rate of the web and can introduce nonuniformity in the web surface properties. These and other such approaches also present problems in that the spray nozzles can drip onto the web or otherwise tend to wet the paper in spots or unevenly, resulting in poor efficiency and surface discontinuities in the rewetting, drying and calendering steps, as well as other operational problems.
It is therefore an object of the invention is to improve the efficiency, uniformity and product quality of drying and/or calendering steps in a papermaking process.
A further object of the invention is to provide a more effective method for conditioning paper and paperboard products prior to rewetting the products.
Yet another object of the invention is to increase the drying efficiency of a papermaking process.
Another object of the invention is to provide a method for conditioning a paper or paperboard product for calendering which reduces operational problems associated with prior methods, and improves surface finishing.
Another object of the invention is to provide an efficient means of cross-directional moisture profiling of a paper or paperboard product on a papermaking machine.